1. Field of Invention
The present invention is related to a cast alloy used for the production of a permanent magnet, which contains rare-earth elements, and to a method for producing the cast alloy. The present invention is also related to a method for producing a rare earth magnet.
2. Description of Related Art
The production amount of rare earth magnets is steadily increasing along with miniaturization and Derformance enhancement of electronic appliances. In particular, the production amount of NdFeB magnets is continuously increasing, because it is superior to the SmCo magnet in the aspects of high performance and low material cost. Meanwhile, demand for the NdFeB magnets, performance of which has been further enhanced, is increasing.
The ferromagnetic phase of the NdFeB magnet, which plays an important role in realizing the magnetic properties, is the R.sub.2 T.sub.14 B phase. This phase is referred to as the main phase. There is also present in the NdFeB magnet a non-magnetic phase, which includes rare earth elements, such as Nd or the like, in high concentration. This phase is referred to as the R-rich phase and also plays an important role as follows.
(1) The R-rich phase has a low melting point and hence is rendered to a liquid phase in the sintering step of the magnet production process. The R-rich phase contributes therefore to densification of the magnet and hence enhancement of magnetization.
(2) The R-rich phase eliminates the defects of the grain boundaries of the R.sub.2 T.sub.14 B phase, which defects lead to the nucleation sites of reversed magnetic domains. The coercive force is thus enhanced due to decreasing to the nucleation sites.
(3) Since the R-rich phase is non-magnetic, the main phases are magnetically isolated from one another. The coercive force is thus enhanced.
It will be understood from the roles mentioned above that, when the dispersion of the R-rich phase is insufficient to cover the grain boundaries of the main phases, local reduction of the coercive force occurs at the non-covered grain-boundaries, and hence the squareness ratio of the magnet is impaired. Furthermore, since the sintering properties are impaired, the magnetization and hence the maximum energy product are lowered.
Meanwhile, since the proportion of the R.sub.2 Fe14B phase, i.e., the ferromagnetic phase, should be increased in the high-performance magnet, the volume fraction of the R-rich phase inevitably decreases. In many cases, however, such attempted increase in the fraction of R.sub.2 Fe.sub.14 B does not necessarily attain high performance, because the local insufficiency or the R-rich phases is not solved. A number of studies have, therefore, been published on how to provide a method for preventing the performance reduction due to the insufficient R-rich phase. They are roughly classified into two groups.
One group proposes to supply the main R.sub.2 Fe .sub.14 B phase and the R-rich phase from separate alloys, respectively. This proposal is generally referred to as the two-alloy blending method. An alloy magnet having a particular composition can be produced by the two-alloy blending method using the two alloys, composition of which can be selected in a wide range. Particularly, one of the alloys, i.e., the alloy for supplying the R-rich phase, can be selected from a large variety of compositions and can be produced by various methods. Several interesting results have accordingly been reported.
For example, an amorphous alloy, which is rendered to a liquid phase at the sintering temperature, can be used as one of the alloys for supplying the grain-boundary phase (hereinafter referred to as "the boundary phase alloy"). In this case, since the amorphous alloy is under a non-equilibrium state, the Fe content of this alloy is adjusted to a higher level than that of the ordinary R-rich phase composition. When a magnet is to be produced by using the amorphous boundary-phase alloy, the mixing ratio of the boundary-phase alloy can be made high corresponding to high Fe content of the amorphous boundary phase alloy. As a result, when the R-rich phases are formed at the sintering steps, they are well dispersed and hence the magnetic properties are successfully enhanced. Furthermore, the amorphous alloy can effectively suppress the powder oxidation (E. Otsuki, T. Otsuka and T. Imai, 11th International Workshop on Rare Earth Magnet and Their Application Vol. 1, p 328 (1990)).
According to another report, a high-Co alloy is used as the boundary phase alloy to successfully prevent the powder oxidation (M. Honshima and K. Ohashi, Journal of Materials Engineering and Performance, Vol. 3(2), April 1994, p218-222).
The other group proposes the strip casting of the final composition alloy. This method realizes a higher cool-no rate than by the conventional metal-mold casting method and hence enables to finely disperse the R-rich phases in the alloy structure produced. Since the R-rich phases are finely dispersed in the cast alloy, their dispersion after crushing and sintering is also excellent so as to successfully improve the magnetic properties (Japanese Unexamined Patent Publications Nos. 5-222,488 and 5-295,490).
Apart from the discussions hereinabove, since the volume fraction of R.sub.2 T.sub.14 B phase is high in the high-performance magnet, its composition becomes close to the stoichiometeric R.sub.2 T.sub.14 B composition. The .alpha.--Fe is liable to form. The .alpha.--Fe in the powder incurs reduction in crushing efficiency in the magnet production. If the .alpha.--Fe remains in the magnet after sintering, the magnet performance is lowered. The .alpha.--Fe must, therefore, be diminished by means of homogenizing heat-treatment of an ingot for a long period of time, if the ingot is produced by the conventional metal-mold casting. The strip casting method is advantageous over the metal-mold casting method, because the precipitation of .alpha.--Fe is suppressed by means of increasing the solidification rate and hence super-cooling the alloy to beneath the peritectic-reaction temperature.
The two-alloy blending method and the strip casting method can be so combined that the main-phase alloy and an alloy with low R content are strip cast. Even in this case, although the R content is so low as to form .alpha.--Fe, the effects of the strip casting, i.e., the suppression of .alpha.--Fe formation and the enhancement of crushing efficiency, are recognized.
When the alloy having a relatively low R content and containing R.sub.2 T.sub.17 is used in the two-alloy blending method, the R content of the main-phase alloy is correspondingly high. Even if the main-phase alloy is cast by the conventional metal-mold casting method, the formation amount of .alpha.--Fe is considered to be small. When such main-phase alloy is cast by the strip casting method, since .alpha.--Fe formation is thoroughly suppressed, extremely good crushing property and good grain dispersion are attained. The strip casting combined with the two-alloy blending method also improves the dispersion of the R-rich phases (Japanese Unexamined Patent Publication No. 7-45,413).